The present invention relates to a fluorescent lamp operating apparatus containing an inverter circuit.
As a conventional fluorescent lamp operating apparatus, such a series inverter as disclosed in Japanese Laid-Open Patent Application No. 10-162983 is known. The applicant of the present invention presented a fluorescent lamp operating apparatus as shown in FIG. 9 in Japanese Patent Application No. 11-161874.
The fluorescent lamp operating apparatus shown in FIG. 9 comprises an AC power supply 1, a noise-proof capacitor 2, a rectifier circuit 3, a smoothing capacitor 4, FETs 5, 6, a first resonance capacitor 7, a fluorescent lamp 8, a preheat capacitor 9, a choke coil 10, a trigger capacitor 14, Zener diodes 15, 16, a second resonance capacitor 17 and a resistance 19. In the structure shown in FIG. 9, the resonance capacitor 7, the fluorescent lamp 8, the choke coil 10 and the source and drain terminals of the P-type FET 6 as the first switching element are connected in series in that order.
The smoothing capacitor 4 is connected between one end of the resonance capacitor 7 and the drain terminal of the FET 6 so as to form a closed circuit. The N-type FET 5, which is the second switching element, is connected between the junction of the choke coil 10 and the FET 6 and the junction of the resonance capacitor 7 and the smoothing capacitor 4. The input terminals of the rectifier circuit 3 are connected to the AC power supply 1 via the noise-proof capacitor 2. The preheat capacitor 9 is connected between a pair of electrodes 8A and 8B of the fluorescent lamp 8 on the side opposite to the power supply 1. All these components together form a high-frequency inverter circuit.
The high-frequency inverter circuit first obtains a direct current by rectifying and smoothing commercial AC power supplied from the AC power supply 1. In this circuit structure, the AC power supply 1, the noise-proof capacitor 2, the rectifier circuit 3 and the smoothing capacitor 4 together compose a DC power supply 22. Then, the obtained direct current is entered into a serial circuit of the switching elements (N-type FET 5 and P-type FET 6) which are connected in parallel between the smoothing capacitor 4 and the fluorescent lamp 8 and oscillate at radio frequencies. After this, the current is entered into an LC resonance circuit composed of the choke coil 10 and the resonance capacitor 7 which is connected to the N-type FET 5 and further to the fluorescent lamp 8 in series. Thus, the inverter circuit shown in FIG. 9 generates high-frequency electric power.
As means for starting the high-frequency inverter circuit, the N-type FET 5 and the P-type FET 6 each have a closed loop between the gate and source terminals. The closed loop is composed of a second choke coil 11, the secondary winding 10A of the choke coil 10 and the trigger capacitor 14 connected in series, and one side of the trigger capacitor 14 is connected to the source terminals of the FETs 5, 6. Furthermore, resistances 12, 13 and 19, the Zener diodes 15, 16, the second choke coil 11, the secondary winding 10A of the choke coil 10 and the second resonance capacitor 17 together compose a gate driving circuit 23. The junction of the resistance 12, the second choke coil 11 and the Zener diode 15 is the output terminal of the gate driving circuit 23.
In the fluorescent lamp operating apparatus structured as described above, before the fluorescent lamp 8 is initiated, the AC power supply 1 supplies the noise-proof capacitor 2 with utility AC power to generate a pulsing voltage via the rectifier circuit 3. The current resulting from the pulsing voltage makes the smoothing capacitor 4 be charged until it reaches the power-supply voltage. In addition, the resonance capacitor 7 and the preheat capacitor 9 are charged via the resistance 19, and at the same time, the trigger capacitor 14 is charged via the resistance 12, the second choke coil 11 and the secondary winding 10A of the choke coil 10.
When the charging voltage of the trigger capacitor 14 reaches the threshold voltage in the Zener diode 15, the electric charge of the trigger capacitor 14 is supplied to the gate terminal of the FET 5 so as to turn the FET 5 on. When the FET 5 is thus placed in the ON state, the electric charges of the resonance capacitor 7 and the preheat capacitor 9 flow into the primary winding 10B of the choke coil 10 via the FET 5.
Then, the current flowing through the primary winding 10B of the choke coil 10 develops an inductive voltage in the secondary winding 10A of the choke coil 10. This causes the second choke coil 11 and the capacitor 17 to resonate, thereby making the capacitor 17 have a voltage opposite in direction to the trigger capacitor 14. Then, a reverse-biased voltage is supplied between the gate and the source of the FET 5 so as to turn the FET 5 off. At the same time, a forward-biased voltage is supplied between the gate and the source of the FET 6 to turn the FET 6 on.
When the FET 6 is thus placed in the ON state, the current flows from the smoothing capacitor 4 through the closed circuit composed of the resonance capacitor 7, the fluorescent lamp 8, the choke coil 10 and the FET 6 so as to resonate the primary winding 10B of the choke coil 10, the resonance capacitor 7 and the preheat capacitor 9. At this moment, the current flowing in the reverse direction through the primary winding 10B of the choke coil 10 develops an inverse inductive voltage at the secondary winding 10A of the choke coil 10, which resonates the second choke coil 11 and the capacitor 17, thereby making the capacitor 17 have a voltage in the opposite direction. As a result, a reverse-biased voltage is supplied between the gate and the source of the FET 6 to turn the FET 6 off. Later, a forward-biased voltage is supplied between the gate and the source of the FET 5 to turn the FET 5 back on. Hereafter, the above-described operations are repeated to turn on and off the FET 5 and the FET 6 alternately.
The above-mentioned current heats the electrodes 8A, 8B while flowing through the preheat electrode of the fluorescent lamp 8. At the same time, a large voltage is placed by resonance between the electrodes of the fluorescent lamp 8, which increases the temperature of the electrodes so as to start a discharge from the state where the impedance between the electrodes of the fluorescent lamp 8 is infinity. Once the discharge is started, the impedance between the electrodes of the fluorescent lamp 8 drops suddenly, so that an abrupt large current flows from the power line through the fluorescent lamp 8 (this phenomenon is hereinafter referred to as a breakdown, and the large current is referred to as a breakdown current). When a breakdown occurs, the impedance decreases enough to be in a normal stable lighting condition.
The inventors of the present invention have found through experiments that in a fluorescent lamp operating apparatus like this, there are cases where a sudden drop in the lamp impedance at a breakdown causes an extremely large and abrupt breakdown current to flow through the power switching elements, and this inrush current breaks the power switching elements. Furthermore, in the electrodes 8A and 8B of the fluorescent lamp 8, an abrupt and large breakdown current at start-up causes electrons to be concentrated to form a heat spot, thereby to increase local heating. This becomes an issue because it may lead to a disconnection of the electrodes to seriously damage the flashing life characteristics of the fluorescent lamp.
The present invention has been contrived in view of these aspects, with a main object of providing a fluorescent lamp operating apparatus having a fluorescent lamp whose flashing life characteristics have been improved in a simple circuit structure.
A fluorescent lamp operating apparatus of the present invention comprises: a power switching element electrically connected to a fluorescent lamp having a pair of electrodes; and means for limiting a current flowing through said power switching element by controlling conductance of said power switching element so as to prevent said power switching element from being broken by a large current flowing from a power line through said fluorescent lamp when an impedance between said pair of electrodes decreases from infinity at start-up of said fluorescent lamp.
Another fluorescent lamp operating apparatus of the present invention comprises: a top-side transistor and a down-side transistor connected in series between output terminals of a direct current power supply; a serial circuit composed of a capacitor, a fluorescent lamp having a pair of electrodes, a preheat capacitor connected in parallel with said fluorescent lamp, and an inductor, said serial circuit being connected between an output terminal of said direct current power supply and a junction of said top-side transistor and said bottom-side transistor; a gate driving circuit for turning on and off said top-side transistor and said bottom-side transistor alternately in accordance with a current flowing through said fluorescent lamp; and resistances connected between respective gate terminals of said top-side transistor and said bottom-side transistor and said gate driving circuit.
In an embodiment, said direct current power supply comprises an alternating current power supply, a rectifier circuit connected to said alternating current power supply, a smoothing capacitor connected between output terminals of said rectifier circuit, and said top-side transistor and said bottom-side transistor are connected in series between the output terminals of said rectifier circuit.
In an embodiment, said top-side transistor and said bottom-side transistor are an N-type transistor and a P-type transistor, respectively.